Optical switch using rare earth doped glass

ABSTRACT

The present invention provides an optical switch including a loss element having a signal loss, and a rare earth doped gain element optically connected in series with the loss element. The rare earth doped gain element is operable to produce a signal gain. The signal gain and the signal loss are about equal. The present invention also provides a method of optical switching including optically connecting a loss element in series with a rare earth doped gain element and passing an optical signal through the loss element and the gain element. The loss element attenuates the optical signal by a first amount. The method further includes selectively applying an optical pump to the gain element to perform the switching, the gain element amplifying the optical signal by the first amount in response to the optical pump.

This application is a divisional of U.S. patent application Ser. No.10/820,098, entitled “Optical Switch Using Rare Earth Doped Glass,”filed on Apr. 7, 2004, the entire disclosure of which is herebyincorporated herein by reference.

FIELD OF THE INVENTION

The technical field of this disclosure is optical components,particularly optical switches in rare earth doped glass.

BACKGROUND OF THE INVENTION

Optical on/off switches are used in optical systems for variousfunctions, such as add/drop multiplexers and amplitude modulators. Aprimary use of on/off switches is to modulate amplitude of continuouswave signals. Additionally, an on/off switch can block of a signal atsome location when a broadcast signal is not meant to be received atthat location.

Switches often incorporate mirrors or other movable blocking mechanisms.The signal is blocked when a mirror or beam is moved into the path of anoptical beam propagating from one waveguide to another. When the mirroris moved out of the path of an optical beam, the beam is coupled to thesecond waveguide. Moving parts may become stuck in an on or off positionso that the signal is permanently blocked or coupled. Material fatigueafter extended use also causes failure of moving parts used in on/offswitches. The properties of a material forming a micro-electromechanicalsystem (MEMS) hinge, for example, may change after hundreds ofrotations, degrading the range of motion available from the hinge.

It would be desirable to have an optical switch that would overcome theabove disadvantages.

SUMMARY OF THE INVENTION

The present invention is an optical switch with no moving parts. Theoptical switch has a loss element and a gain element. The gain elementis formed from a waveguide doped with at least one species of rare earthions. The switch operates to transmit an optical signal from an opticalsource when pump power is provided to a rare earth doped gain element.The switch will operate to absorb the optical signal when pump power isnot provided to a rare earth doped gain element. Turning the pump poweron and off switches the optical signal on and off.

One aspect of the present invention provides an optical switch includinga loss element having a signal loss and a rare earth doped gain elementoptically connected in series with the loss element. The rare earthdoped gain element is operable to produce a signal gain, with the signalgain and the signal loss being about equal.

Another aspect of the present invention provides a method of opticalswitching including optically connecting a loss element in series with arare earth doped gain element and passing an optical signal through theloss element and the gain element. The loss element attenuates theoptical signal by a first amount. The method further includesselectively applying an optical pump to the gain element to perform theswitching, the gain element amplifying the optical signal by the firstamount in response to the optical pump.

The foregoing and other features and advantages of the invention willbecome further apparent from the following detailed description of thepresently preferred embodiments, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the invention, rather than limiting the scope of theinvention being defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an optical switch in accordance with thepresent invention;

FIG. 2 shows the optical intensity of a signal passing through anexemplary optical switch in the ON and the OFF state;

FIG. 3 shows a schematic of the loss element;

FIG. 4 shows a schematic of the gain element;

FIG. 5 shows an energy level diagram for a three level system for anexemplary erbium ion Er³⁺;

FIG. 6 shows measured and theoretical gain spectra for a gain elementmade in accordance with the present invention;

FIG. 7 shows an absorption coefficient curve for a loss element made inaccordance with the present invention;

FIG. 8 shows a schematic of an optical switch in accordance with thepresent invention operating in a reverse mode;

FIG. 9 shows a schematic of an optical switch in accordance with thepresent invention;

FIG. 10 shows a schematic of optical switches in parallel in accordancewith the present invention.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 shows a top view of an optical switch 10, which is composed of aloss element 20 and a gain element 30, both on a supporting substrate15. An optical pump source 50 is mounted off the supporting substrate15. In an alternative embodiment, the optical pump source 50 is an edgeemitting laser diode attached to or formed within the substrate 15. Theloss element 20 has an input endface 21 and an output endface 22. Theloss element 20 attenuates input optical signal 60, which is coupled tothe input endface 21, and generates attenuated optical signal 61. Theoutput endface 22 is optically coupled to coupling element 40, whichtransmits attenuated optical signal 61. The gain element 30 has an inputendface 31 and an output endface 32. The coupling element 40 couplesattenuated optical signal 61 into the input endface 31 of the gainelement 30. The gain element 30 generates amplified output opticalsignal 70.

The optical pump source 50 has an ON state and an OFF statecorresponding to the ON state and an OFF state of the optical switch 10.In the ON state, the optical pump source 50 emits an optical pump 51. Inthe OFF state, no optical pump is emitted. When the optical switch 10 isin the ON state, the optical pump 51 is coupled to the gain element 30,which places the gain element 30 in the ON state. The attenuated opticalsignal 61 is amplified as it passes through the gain element 30 in theON state and the amplified output optical signal 70 exits the gainelement 30 at output endface 32.

The loss element 20 attenuates the input optical signal 60 to producethe attenuated optical signal 61, which is lower in intensity than theinput optical signal 60 by a signal loss amount. When the gain element30 is ON, the gain element 30 amplifies the attenuated optical signal 61to produce the amplified output optical signal 70, which is greater inintensity than the attenuated optical signal 61 by a signal gain amount.Because the absolute values of the signal gain and the signal loss areabout equal, the amplified output optical signal 70 has about the sameintensity as the input optical signal 60. Values of the signal gain andthe signal loss that differ by a few decibels will be regarded as beingabout equal, although in particular applications a much largerdifference is acceptable. When the gain element 30 is OFF, the gainelement 30 further attenuates the attenuated optical signal 61 and theoutput from the optical switch 10 is negligible in intensity.

Possible coupling mechanisms by which the input optical signal 60 iscoupled to the loss element 20 and by which the coupling element 40optically communicates with the loss element 20 and the gain element 30include a lens coupling, an end fire coupling, diffractive coupling, agrating coupler, a fused optical fiber coupler, and combinationsthereof. Possible coupling mechanisms by which the optical pump 51 iscoupled to the gain element 30 include a diffractive coupler, a y-branchcoupler, a directional coupler, a grating coupler, a fused optical fibercoupler, or combinations thereof. The coupling device 40 is a fiber oran optical waveguide. In one embodiment, the coupling device 40 isomitted and the gain element 30 and the loss element 20 are directlycoupled by end fire coupling, lens coupling, or a combination thereof.

FIG. 2, in which like elements share like reference numbers with FIG. 1,shows the optical intensity of a signal passing through an exemplaryoptical switch 10 in the ON and the OFF state. The signal gain in thegain element 30 is designed to be equal to the signal loss in the losselement 20. In this example, the loss element 20 produces a signal lossof 15 dB. The gain element 30 produces a signal gain of 15 dB when thegain element 30 is ON and a loss of 15 dB when the gain element 30 isOFF. The optical switch 10 is ON when the optical pump 51 is coupled tothe gain element 30 and the optical switch 10 is OFF when the opticalpump 51 is not coupled to the gain element 30.

In an example, the input optical signal 60 is 1 mW or 0 dBm at the inputendface 21 of loss element 20. After propagating through the losselement 20 the input optical signal 60 is attenuated by 15 dB and has anoptical intensity of about 30 μW or −15 dBm. The attenuated opticalsignal 61 is emitted from output endface 22 of loss element 20 andpropagates without appreciable loss or gain through the coupling element40 to the input endface 31 of the gain element 30.

Attenuated optical signal 61 propagates through the gain element 30 andexperiences a 15 dB gain when the optical switch 10 is in the ON state.Line 77 shows how the attenuated optical signal 61 gains intensity as itpasses through the gain element 30 with an optical pump 51. An amplifiedoutput optical signal 70 is emitted from output endface 32 of at thesame optical intensity of 1 mW or 0 dBm as the input optical signal 60.The optical signal is amplified by a gain that offsets the signal lossof the attenuated optical signal 61 in the loss element 20, since forthis exemplary optical switch 10 the signal gain of gain element 30equals the absolute value of the signal loss of loss element 20. Theamplification in the gain element results from stimulated emission fromoptically pumped rare earth ions.

The gain element 30 further attenuates optical signal 61 when no opticalpump 51 is propagating in the gain element 30. Attenuated optical signal61 propagates through the gain element 30 and experiences a −15 dB losswhen optical pump 51 is not coupled to the gain element 30. Line 78shows how the attenuated optical signal 61 loses intensity as it passesthrough the un-pumped gain element 30. Attenuated output optical signal70 is emitted from output endface 32 with an optical intensity of 1 μWor −30 dBm. This exemplary optical switch 10 has an on/off ratio of1000/1. The attenuated output optical signal 70 can be attenuated withrespect to the intensity of input optical signal 60 within the range of−10 dB to more than −90 dB depending on the switch design.

FIG. 3 shows the loss element 20. In this embodiment, the loss element20 is a waveguide composed of a core 23 heavily doped with at least onespecies of rare earth ion (not shown), a cladding 24, an input endface21, and an output endface 22. The core 23 is surrounded at least in partby cladding 24. The cladding 24 has a cladding index of refraction,which is less than the core index of refraction of the core 23. Thecladding 24 may also be heavily doped with at least one species of rareearth ion. The waveguide of loss element 20 is connected to receive theinput optical signal 60. The loss element 20 supports propagation of oneor more optical modes of radiation above a certain wavelength. In analternative embodiment, the loss element 20 is a ridge-loaded waveguideformed by disposing a lower index material having a desired width andlength on a planar waveguide heavily doped with at least one species ofrare earth ion.

As the input optical signal 60 propagates through the loss element 20,it is absorbed by the un-pumped rare earth ions in the loss element 20and is thereby attenuated. The attenuated optical signal 61 exits losselement 20 at the output endface 22.

In an alternative embodiment, the loss element 20 is an un-dopedwaveguide, i.e., a waveguide which is not doped with a rare earth ion,although the waveguide may be doped with other elements as desired. Thematerial or combination of materials forming the loss element 20 absorbslight at the wavelength of the input optical signal 60 while supportingpropagation of one or more optical modes of radiation at thatwavelength. The optical pump 51 may be coupled into the input endface 21of loss element 20 when the un-doped waveguide of the loss element 20 isnot absorbing or is minimally absorbing at the wavelength of the opticalpump 51.

In another alternative embodiment, the loss element 20 is a length ofabsorbing material, such as a neutral density filter, which absorbslight at the wavelength of the input optical signal 60. The optical pump51 may be coupled into the input endface 21 of the loss element 20 whenthe length of absorbing material of the loss element 20 is not absorbingor is minimally absorbing at the wavelength of the optical pump 51.

FIG. 4 shows the gain element 30. The gain element 30 is a waveguidecomposed of a core 33 heavily doped with at least one species of rareearth ion (not shown), a cladding 34, an input endface 31, and an outputendface 32. The core 33 is surrounded at least in part by cladding 34.The cladding 34 has a cladding index of refraction, which is less thanthe core index of refraction of the core 33. The cladding 34 may also beheavily doped with at least one species of rare earth ion. The waveguideof gain element 30 is connected to receive an attenuated optical signal61 and an optical pump 51. The gain element 30 supports propagation ofone or more optical modes of radiation above a certain wavelength. In analternative embodiment, the gain element 30 is a ridge-loaded waveguideformed by disposing a lower index material having a desired width andlength on a planar waveguide heavily doped with at least one species ofrare earth ion. The loss element 20 and the rare earth doped gainelement 30 are in optical communication and the rare earth doped gainelement 30 has a gain responsive to the optical pump 51.

The gain element 30 amplifies the attenuated optical signal 61 when theoptical switch 10 is ON and attenuates the attenuated optical signal 61when the optical switch 10 is OFF. In the ON state, the optical pump 51excites the rare earth ions (not shown) in the core 33. Theamplification of attenuated optical signal 61 is a result of excitationof rare earth ions in the gain element 30 by the optical pump 51.

The heavy rare earth doping of the core 33 amplifies the attenuatedoptical signal 61 as attenuated optical signal 61 propagates through thegain element 30 of the optical switch 10 when the optical switch 30 inthe ON state. The amplified output optical signal 70 and the opticalpump 51 exit the gain element 30 at the output endface 32. The intensityof amplified output optical signal 70 equals the intensity of the inputoptical signal 60 when the signal gain of gain element 30 equals theabsolute value of the signal loss of loss element 20.

The optical switch 10 is OFF when the optical pump 51 is OFF, because nooptical pump propagates in the core 33 to excite the rare earth ions(not shown). The heavy doping of rare earth ions in the core 33 furtherattenuates attenuated optical signal 61 as the attenuated optical signal61 propagates through the gain element. When the optical switch 10 isOFF, the output optical signal 70 has a very low intensity which canrange from one tenth ( 1/10) to less than one thousandth ( 1/1000) ofthat of the input optical signal 60, depending on the particular designof the optical switch 10.

The loss element 20 and the gain element 30 are shown with identicalstructures in the present example for clarity, although in otherembodiments the loss element 20 is an un-doped waveguide or a neutraldensity filter. The loss element 20 and the gain element 30 arewaveguides with cores and claddings. The cladding materials need nothave the same index of refraction on all sides of the core. The claddingindex of refraction, the core index of refraction, and the geometry ofthe core (the width and the thickness), all affect the modal structureof light at a wavelength propagating in the waveguide. Telecommunicationsystems generally use single mode fibers to transmit optical signals inthe wavelength region of 1.5 μm, so it is desirable that the losselement 20 and the gain element 30 of the optical switch 10 are singlemode at the wavelength of 1.5 μm for telecommunications applications. Inone embodiment, the optical signal 60 to be attenuated has a wavelengthin the range of 1.5 μm to 1.7 μm.

Glasses host the rare earth dopants in the core 22 and cladding 24 ofthe loss element 20 and in the core 33 and cladding 34 of the gainelement 30. Glasses are covalently bonded molecules in the form of adisordered matrix with a wide range of bond lengths and bond angles.Phosphate, tellurite, and borate glasses can accept a high concentrationof rare earth ions, including Er³⁺ ions. The high solubility of rareearth ions in these glasses permits high signal gain in the gain element30 and the high signal loss in the loss element 20. Typically, the cores23 and 33 are formed of phosphate, tellurite, or borate glasses heavilydoped with at least one species of rare earth ion. The claddings 24 and34 are typically formed of the same type of glasses as the cores 23 and33. When claddings 24 and 34 are not doped with rare earth dopants, thedopants in the cores 23 and 33 ensure the index of refraction of thecores 23 and 33 is higher than the index of refraction of the claddings24 and 34.

In an alternative embodiment, phosphate, tellurite, or borate glassesheavily doped with at least one species of rare earth ion form the cores23 and 33 and the claddings 24 and 34. When the cores 23 and 33 and thecladdings 24 and 34 are identically doped with rare earth ions, anadditional dopant is injected or diffused into the cores 22 and 32 toincrease the index of refraction of the cores 23 and 33. In oneembodiment, a patterned diffusion of silver ions is used to increase theindex of refraction of the cores 23 and 33.

When the core and the cladding are doped with different species of rareearth ions, the dopants are selected so that the core has a higher indexof refraction than the cladding. In this way, the core 23 of the losselement 20 supports at least one mode of input optical signal 60 and thecore 33 of the gain element 30 supports at least one mode of attenuatedsignal 60 and optical pump 51.

The loss within the loss element 20 of the optical switch 10 resultsfrom absorption of the input optical signal 60 by the rare earth ions.In alternative embodiments, the loss element 20 is a neutral densityfilter or an un-doped waveguide, which absorb light at the wavelength ofthe input optical signal 60 and the loss results from their particularabsorption characteristics.

The amplification within the gain element 30 of the optical switch 10results from excitation of the rare earth ions by the optical pump 51.Rare earth ions or lanthanides range from lanthanum with an atomicnumber of 57 to lutetium with an atomic number of 71, and are lanthanum,cerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,and lutetium.

Various rare earth doping concentrations in the cores 23 and 33 can beused in optical switch 10. In one embodiment, the cores 23 and 33 aredoped with Er³⁺ in the range of 5 to 75 wt %. In another embodiment, thecores 23 and 33 are doped with Er³⁺ in the range of 5 to 30 wt %.Typically, the cores 23 and 33 are doped with Er³⁺ in the range of 7 to9 wt %. This dopant level is high enough to produce sufficient signalloss in a loss element 20 less than a few centimeters long andsufficient signal gain in a gain element 30 less than a few centimeterslong.

Phosphate, tellurite, or borate glasses accept 5 to 75 wt % of a singlespecies of rare earth ion without precipitation. However, ion clustersmay form at higher levels of the dopant. Ion clusters promote ionself-interactions so that the absorbed optical pump 51 is exchangedbetween clustered ions and does not promote amplification of theattenuated optical signal 61. Thus, ion clusters deplete the pump poweravailable for amplification as pump power is absorbed to excite ionself-interactions. Amplification is quenched if too many ion clustersform. In order to prevent the formation of ion clusters, a secondspecies of rare earth ion is added as a second dopant to the glass.

If the dopant level of the second species is about equal to that of thefirst species, the second species will decrease the probability of ioncluster formations of either species. A rare earth ion of either speciesis half as likely to be positioned next to a rare earth ion of the samespecies. The probability of large ion clusters forming is reduced evenmore. Thus, mixing different species of rare earth ions reduces ioncluster formations of either species.

In addition, the absorption cross section of the optical pump 51 inglass doped with rare earth ions is larger than the absorption crosssection of the optical pump 51 of the species alone. By doping aphosphate, tellurite or borate glass with two or more species of rareearth ion, more optical pump 51 is absorbed to provide gain ofattenuated optical signal 61 within the gain element 30 of opticalswitch 10. In addition, doping a phosphate, tellurite or borate glasswith two or more species of rare earth ion results in a larger portionof input optical signal 60 being absorbed within the loss element 20 ofoptical switch 10. This increases attenuation of the input opticalsignal in the rare earth loss element 20. This also increasesamplification of the attenuated optical signal in the rare earth gainelement 30 when pump power 51 is coupled to the rare earth gain elementand increases attenuation of the attenuated optical signal 61 in therare earth gain element when pump power 51 is not coupled to the rareearth gain element.

In one embodiment, the core 23 of the loss element 20 of optical switch10 is doped with Er³⁺ in the range of 5 to 75 wt % and Yb³⁺ in the rangeof 7 to 35 wt %. The core 33 of gain element 30 of optical switch 10 isdoped with Er³⁺ in the range of 5 to 75 wt % and Yb³⁺ in the range of 7to 35 wt %. In another embodiment, the core 23 of the loss element 20 ofoptical switch 10 is doped with Er³⁺ in the range of 5 to 30 wt % andYb³⁺ in the range of 7 to 35 wt %. The core 33 of gain element 30 ofoptical switch 10 is doped with Er³⁺ in the range of 5 to 30 wt % andYb³⁺ in the range of 7 to 35 wt %. Typically, the core 23 of the losselement 20 is doped with Er³⁺ in the range of 7 to 9 wt % and with Yb³⁺in the range of 11 to 13 wt %, while the core 33 of gain element 30 isdoped with Er³⁺ in the range of 7 to 9 wt % and with Yb³⁺ in the rangeof 11 to 13 wt %.

FIG. 5 shows an energy diagram of the three level system for anexemplary erbium ion Er³⁺. Ionization of the rare earth ions normallyforms a trivalent state. For example, the rare earth ion erbium (Er³⁺)has a three level system with stimulated emission transitions atwavelengths of 0.80 μm, 0.98 μm, and 1.55 μm. An optical pump power atwavelength of 0.98 μm excites the erbium ion from the ground state E₀ tothe energy level E₂, as illustrated by arrow 55. The ion experiences arapid decay from energy level E₂ to the energy level E₁, as illustratedby arrow 56. The erbium ion Er³⁺ drops from the E₁ energy level to theground state E₀, as illustrated by arrow 57, emitting a photon 71 havinga wavelength of about 1.55 μm. The emitted photon 71 has a probabilityof being emitted within a range of wavelengths centered about thewavelength of 1.55 μm due to the fine structure of the ion energy levels(not shown).

The higher the level of doping of the rare earth ions in the losselement and the gain element, the higher the attenuation andamplification in the loss element and the gain element, respectively.The higher the attenuation and amplification, the shorter the opticalswitch needs to be for a desired ON/OFF ratio. The attenuated opticalsignal at a wavelength within the gain spectrum of an exemplary rareearth ion may be designed to propagate with an optical pump power in thegain element 30. When the optical pump 51 is at the wavelength needed toexcite the rare earth ions, the attenuated optical signal 61 will beamplified after propagating a short distance by the photons 71. Thephotons 71 are emitted by a stimulated process as the excited rare earthions drop into the ground state E₀.

FIG. 6 shows the theoretical gain spectrum 47 of a gain element formedfrom phosphate glass heavily doped with erbium and ytterbium. In thisexample, the dopant level is about 8 wt % Er³⁺ and about 12 wt % Yb³⁺.Such glass is available from Schott Corporation (number IOG-1). FIG. 6also shows the measured gain spectrum 48 for an actual gain element.These experimental results show that amplification is possible in ashort gain element. The core of the gain element was formed in the 8 wt% Er³⁺ and 12 wt % Yb³⁺ doped phosphate glass by diffusion of silverions. The core dimensions were 13 μm wide and 5 μm thick. Air formed thetop cladding layer for the core and the phosphate glass substrate formedthe bottom and side cladding. A 3 mm length of the gain elementamplified an input signal at 1.534 μm by 4 dB when an input optical pumppower of less than 180 mW at 0.974 μm was coupled to the gain element.As the optical pump power is increased above 180 mW at 0.974 μm theamplification increases to more than 6 dB. Increasing the optical pumppower, increasing the gain element length, or increasing both theoptical pump power and the gain element length increases theamplification as required for a particular application, such asincreasing the amplification to 15 dB. Other changes in the gain elementalso increase the amplification, such as applying an encapsulating topcladding layer reduces the scattering loss and increases the overalltransmission in the gain element.

FIG. 7 shows the absorption coefficient in dB/mm as a function ofwavelength for phosphate glass doped with 8 wt % Er³⁺ and 12 wt % Yb³⁺.The peak absorption of more than 2.0 dB per mm at the wavelength of1.534 μm wavelength. For a loss element 20 formed in the same manner asthe gain element 30 described above in conjunction with FIG. 6 the losswill be about 2 dB per mm for a signal at a wavelength of 1.534 μm. Theloss would be similar in gain element 30 without the optical pump 51applied.

When the loss element 20 is a neutral density filter or an absorbingwaveguide, the material comprising the filter or waveguide is chosen forits absorption spectral characteristics. The loss of input opticalsignal 60 after propagating through loss element 20 is a function of thepropagation length-absorption coefficient product at the wavelength ofinput optical signal 60. The propagation length-absorption coefficientproduct is used in the design of the loss element 20 to provide a lossthat is offset by the gain when the gain element 30 is in the ON state.

FIG. 8, in which like elements share like reference numbers with FIG. 1,shows a top view of an alternative embodiment of an optical switch 110in which the input optical signal 160 is coupled to the gain element 30instead of the loss element 20. The embodiment of FIG. 8 is similar tothe embodiment of FIG. 1, except that the input faces and output facesare reversed. A filter 52 is placed between to the output endface 131and coupling element 40 to absorb or reflect optical pump 51. In analternative embodiment, no filter is necessary when the loss element 20is a neutral density filter or an un-doped waveguide, which absorbslight at the wavelength of the input optical signal 160.

The optical switch 110 is in the ON state when the optical pump 51 iscoupled to the gain element 30. The input optical signal 160 is coupledto input endface 132 of the gain element 30, is amplified when passingthrough the gain element 30, and exits the output endface 131 asintermediate signal 162. The intermediate signal 162 passes through thefilter 52 and is coupled to the coupling element 40. The filter 52absorbs or reflects the optical pump 51, so that the optical pump 51 isnot input into the loss element 20 and the loss element 20 does not actas a gain element. The coupling element 40 transmits the intermediatesignal 162 to the input endface 122, where the intermediate signal 162is coupled to the loss element 20. The loss element 20 attenuates theintermediate signal 162, which exits the output endface 121 as outputoptical signal 170.

The optical switch 110 is in the OFF state when optical pump 51 is notcoupled to the gain element 30. The input optical signal 160 is coupledto input endface 132 of the gain element 30, is attenuated when passingthrough the gain element 30, and exits the output endface 131 asintermediate signal 162. The intermediate signal 162 passes through thefilter 52 and is coupled to the coupling element 40. The couplingelement 40 transmits the intermediate signal 162 to the input endface122, where the intermediate signal 162 is coupled to the loss element20. The intermediate signal 162 is additionally attenuated by the losselement 20 and exits output endface 121 as output optical signal 170.When the optical switch 110 is in the OFF state, the input opticalsignal 160 is attenuated in the range of −10 dB to −90 dB or more,depending on the design of optical switch 110. In an alternateembodiment of optical switch 110, the filter 52 is placed between thecoupling element 40 and the input endface 122 of loss element 20.

FIG. 9, in which like elements share like reference numbers with FIG. 1,shows an optical switch 12 in which the gain element 30 and the losselement 20 share a common waveguide 42. The core 43 of waveguide 42 isheavily doped with at least one species of rare earth ion and issurrounded by cladding 44 at least in part. The single waveguide 42 ofoptical switch 12 obviates the need for coupling element 40 of opticalswitch 10 as shown in FIG. 1. The optical switch 12 is a rare earthdoped waveguide 42 connected to receive the optical pump 51 at acoupling region 46, which is located part way along the waveguide 42.The optical pump 51 is coupled to the waveguide 42 in a coupling region46 formed by a Y-branch waveguide 45 of waveguide 42 intersecting thewaveguide core 42. The gain element 30 begins at the coupling region 46where the optical pump 51 enters the single core 43. The optical pumpsource 50 is aligned with and coupled to the Y-branch waveguide 45. Thewaveguide 42 and the branch waveguide 45 are supported by substrate 15.

In one embodiment, the optical pump 51 is coupled to waveguide 42 viathe branch waveguide 45 at the midsection of the waveguide 42. Thisensures that the signal gain in the gain element 30 and the absolutevalue of signal loss in the loss element 20 are approximately equal. Inalternative embodiments, the optical pump 51 is coupled to the couplingregion of waveguide 42 with a diffractive coupler, a directionalcoupler, a grating coupler, or a combination thereof.

FIG. 10 shows a block diagram illustrating use of two optical switches12 in parallel on a substrate 15. Like elements of FIG. 10 share likereference numbers with FIG. 9, with the letters A or B appended todistinguish duplicate elements in FIG. 10. The optical switches 12A and12B are shown schematically as ovals. The output branch 82 may becoupled to an optical fiber in a telecommunications system.

Input optical signals 60A and 60B are coupled into the optical switches12A and 12B, respectively. Input optical pump sources 50A, 50B emitoptical pumps 51A, 51B to place the optical switches 12A, 12B in an ONstate. Y-shaped optical waveguide 80 comprises a first input branch 80A,a second input branch 80B and an output branch 82. The first inputbranch BOA and the second input branch 80B are joined at the Y-junction81 to optically couple to the single output branch 82.

When the optical switch 12A is ON because the optical pump 50A isprovided to the optical switch 12A, and the optical switch 12B is OFF,the input signal 60A is output from the output branch 82 as the outputoptical signal 75. When the optical switch 12B is ON because the opticalpump 50B is provided to the optical switch 12B, and the optical switch12A is OFF, the input signal 60B is output from the output branch 82 asthe output optical signal 75. When both the optical switches 12A and 12Bare ON, both input signals 60A and 60B are output from the output branch82 as the output optical signal 75. In one embodiment, the opticalsignals 60A and 60B are at different wavelengths.

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the scope of the invention. The scope of theinvention is indicated in the appended claims, and all changes that comewithin the meaning and range of equivalents are intended to be embracedtherein.

1. A method of optical switching, comprising: optically connecting aloss element with a gain element doped with at least one species of arare earth ion via a coupling element configured to receive an inputfrom an optical pump; passing an input optical signal through the losselement, the coupling element and the gain element in that order, theloss element generating an attenuated optical signal by attenuating theinput optical signal by a predetermined optical signal loss; andselectively applying the optical pump to the gain element via thecoupling element to perform the optical switching, the gain elementamplifying the attenuated optical signal by a predetermined opticalsignal gain of about equal magnitude to the predetermined optical signalloss in response to the optical pump such that when the gain element isin an ON state an amplified optical output signal has about the sameintensity as the input optical signal and further attenuating theattenuated optical signal in the gain element when the gain element isin an OFF state such that the output of the gain element is negligiblein intensity, the signal power of the optical signal in the couplingelement being less than a signal power of the input optical signal. 2.The method of claim 1, wherein optically connecting comprises aligningan endface of the loss element with an endface of the gain element. 3.The method of claim 1, wherein optically connecting comprises aligningendfaces of the loss element with endfaces of the gain element inparallel.
 4. The method of claim 1, wherein optically connectingcomprises directly coupling the loss element and the gain element viathe coupling element.
 5. The method of claim 1, wherein opticallyconnecting comprises using one of an optical fiber and an opticalwaveguide.
 6. The method of claim 1, wherein passing an optical signalcomprises using one of lens coupling, end fire coupling, diffractivecoupling, a grating, and a fused fiber and combinations thereof tocommunicate the optical signal from a coupling element to the gainelement via the coupling element.
 7. The method of claim 6, whereinpassing an optical signal comprises using one of lens coupling, end firecoupling, diffractive coupling, a grating, and a fused fiber andcombinations thereof to communicate the optical signal from the couplingelement to the gain element.
 8. The method of claim 1, whereinselectively applying the optical pump comprises stimulating emissionfrom optically pumped rare earth ions.
 9. The method of claim 1, whereinoptically connecting comprises coupling single mode waveguides at thewavelength of 1.5 micrometers.
 10. The method of claim 1, whereinoptically connecting comprises coupling single mode waveguides having acore and a cladding, in which the cladding is doped with at least onerare earth ion.
 11. The method of claim 1, wherein optically connectingcomprises coupling single mode waveguides having a core and a cladding,in which the core is doped with Er³⁺.
 12. The method of claim 1, whereinoptically connecting comprises coupling single mode waveguides having acore and a cladding, in which the core is doped with Er³⁺ and Yb³⁺. 13.The method of claim 1, wherein optically connecting comprises insertinga filter between the loss element and the coupling element.
 14. Themethod of claim 13, wherein inserting a filter comprises using a filterthat absorbs light from the optical pump.
 15. The method of claim 13,wherein inserting a filter comprises using a filter that reflects lightfrom the optical pump.
 16. The method of claim 1, wherein opticallyconnecting comprises applying a coupling region that comprises one of adiffractive coupler, a y-branch coupler, a directional coupler, agrating coupler, a fused optical fiber coupler and a combinationthereof.